It is necessary to check the fuel assemblies extracted from the core of the reactor. The conventionally used method consists in measuring the activity released by an assembly when it is plunged in a fluid. But with this method it is only possible to check that the whole assembly is tight. It does not allow to detect which element or elements of a defective assembly is or are leaking.
Numerous methods for non-destructive testing of the fuel elements by using ultrasounds already exist. Among others, there is one (EP-A 0 115 231) which consists in the emission of a train of ultrasonic waves, fed into the sheath from one of its end parts, at a frequency selected so that they travel in the form of Lamb waves; the echoes are then detected and analyzed. Such detection is performed at frequencies whose range corresponds for one fraction to notable absorption by the water possibly contained in the sheath, and for another fraction to a reflection from the end of the spring, especially in case of pressurization.
This solution is perfectly satisfactory when the waves are emitted and received by the transducer placed on the end face of the cap which closes the fuel element. But up to now it has only been possible to control all the elements using such a process when one of the end pieces of the assembly has been previously removed, because of the presence of the braces, of a diameter substantially greater than that of the fuel elements, and the differences of level of the caps in the assembly which have sejourned in the reactor. Unfortunately, the replacement of an end piece, in many assemblies, represents a delicate operation.
To overcome the need of dismantling, devices have also been proposed (FR-A-2 341 183) allowing a transducer to be introduced into the bundle, transversely to the axis thereof, and to be applied against the side wall of the sheath of any one of the fuel elements. By advancing the transducer all the elements of the same network may be checked. The checking time may be further reduced by providing several transducers, carried by a comb shaped piece. But the use of transducers applied against the side wall of the sheath gives less favorable results than that of sensors applied against a cap and leaves unsolved the problem of the masking of some elements by braces; in addition, this use is independent of the position of the water in the element.